I have installed the signal handler as you advised me. but still the exec call terminates the process and i don't know why.

#!/usr/bin/perl -w
use strict;
use Socket;
use Carp;
use MTS;
use MTS qw(prepare_transaction);
use Proc::Daemon;
use File::Temp qw(tempfile);
use Data::Dumper;
use Logger::Syslog;
use JSON;
use File::Find;
use File::Basename;
use Cwd 'realpath';
use POSIX ();
#use FindBin();
use File::Spec::Functions qw(rel2abs catfile);
no warnings 'File::Find';

my $SELF = rel2abs($0);
my @argv = @ARGV;
# Daemonizing
Proc::Daemon::Init unless defined($ENV{"DEBUG"});



use vars qw($mts $qtime $qattr $qsender $qempty $qxtra $qend @template
+ 
        $PORT @rcpts $pid $lock_dir @children $cfg
        $CONFIG_FILE);
$CONFIG_FILE = '/etc/mts/mts.conf';

# 
# Forward Declarations
#
sub spawn;  
sub _dispatcher;
sub _check_spam;
sub _process_request;
sub _build_msg;
sub _enqueue_messages;
sub _prepare_message;
sub _notify;
sub _signal_handler;
sub _recover_transactions;
sub _lock_maildrop_dir;
sub _read_config;
sub _reload_config;

$SIG{CHLD} = \&REAPER;
$SIG{HUP} = \&sigHUP_handler;
#$SIG{__DIE__} = 'IGNORE';
$SIG{ABRT} = \&_signal_handler;
$SIG{TERM} = \&_signal_handler;
$SIG{INT} = \&_signal_handler;

_read_config;
#print Dumper $cfg;

sub sigHUP_handler {
    #_reload_config;
    debug("Restarting daemon with: \$SELF=$SELF and \@argv=@argv ");
    exec($SELF,@argv);
}


# POSIX unmasks the sigprocmask properly
#my $sigset = POSIX::SigSet->new();
#my $action = POSIX::SigAction->new('sigHUP_handler', $sigset, &POSIX:
+:SA_NODEFER);
#POSIX::sigaction(&POSIX::SIGHUP, $action);
#$action = POSIX::SigAction->new('_signal_handler', $sigset, &POSIX::S
+A_NODEFER);
#POSIX::sigaction(&POSIX::SIGTERM, $action);
#POSIX::sigaction(&POSIX::SIGINT, $action);
#POSIX::sigaction(&POSIX::SIGABRT, $action);
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pay attention to the commented code block which uses POSIX. This does the same problem too